It is known to use a capacitance-type transducer for the measurement of engine manifold pressure and other quantities such as fluid level, and position and/or proximity of various items. One known transducer comprises a capsule made up of quartz plates which are assembled in a normally parallel and spaced apart arrangement with capacitor electrodes bonded to the inside facing surfaces to form a pair of capacitors. When the quartz plates are subjected to varying pressure, they flex so as to vary the spacing between the capacitor electrodes and to cause a measurable capacitive change.
To generate an electrical signal representing the capacitance change, the transducer is connected to a demodulator circuit comprising an alternating excitation voltage source, a four-diode bridge circuit, and an output filter capacitor across which a DC voltage is developed in proportion to the capacitance change. A more complete explanation of the quartz capsule pressure transducer may be found in the SAE Technical Paper Series, Paper No. 810374 "Quartz Capsule Pressure Transducer for the Automotive Industry", presented at the International Congress and Exposition in Detroit Feb. 23-27, 1981. The diode-quad demodulator circuit is more fully described in U.S. Pat. No. 3,869,676 issued Mar. 4, 1975 and assigned to the United States of America as represented by the NASA administrator.
There are certain disadvantages associated with the diode-quad bridge circuit, primarily as a result of the use of the diodes. There is, of course, a voltage drop associated with each of the diodes and the cumulative voltage drop limits the effective amplitude of the excitation voltage and, thus, the detection sensitivity. In addition, the diodes introduce a temperature sensitive component into the circuit transfer function which requires compensation in applications where wide temperature swings are encountered and high accuracy is required. One approach to compensation is disclosed in U.S. Pat. No. 4,289,035 issued Sept. 15, 1981 to Chen Y. Lee and assigned to the Bendix Corporation.
Overall, it is generally recognized as difficult to detect small capacitance changes because of the presence of noise due to inherent capacitive components of connectors and other circuit elements, stray fields, and the effect of proximity to other bodies carrying varying charge levels. The diode-quad demodulator circuit has significant advantages but, as explained above, also suffers from various limitations. Therefore there exists a need for improved demodulators for capacitance-type signal generators or transducers.